Soften 2 Tbsp of butter and then spread evenly across a large, rimmed sheet pan. Place pan in a warm (~150°F) oven.

In a large saucepan, combine the remaining stick of butter with the sugar, water, and corn syrup. Bring mixture to a boil over medium heat, stirring occasionally. (See note below on pan size.)

Continue cooking over medium heat (stirring frequently) until temperature reaches 300 on a candy thermometer.

While sugar mixture is heating, combine the baking soda, salt, and peanuts in a small bowl. Set aside. Measure out the vanilla extract and set aside.

Once sugar mixture reaches 300°F, remove saucepan from heat and immediately stir in baking soda, vanilla, salt, and peanuts. (This will cause the mixture to bubble up slightly, so use caution.)

Immediately pour mixture onto greased, warm sheet pan; use a spatula to spread the mixture into a thin layer about 1/4" thick.

Quickly scatter the peanut M&Ms on top of the mixture; lightly press M&Ms into brittle. (Use caution here as the brittle mixture is still very hot!)

Let brittle cool for 30-45 minutes at room temperature; break into pieces before serving.